The phytoplankton in Lake Baikal included autotrophic picoplankton (APP), nano- and microphytoplankton. The phytoplankton biovolume, the Chla and other phytoplankton pigments were distributed very heterogeneously in Lake Baikal, thereby indicating variations of both phytoplankton abundance and composition. Differences were found between the open basins (South, Centre, and North) as well as between the near-shore (Maloe More) and river-inflow (Selenga Delta, Barguzin Bay) sites.

Regional distribution of APP, nano- and microphytoplankton: The whole lake median of the total phytoplankton biovolume was 0.61 mm³ L-1, of which 78 % was nano- and microphytoplankton and 22 % APP. The median APP biovolume over the whole lake was 0.097 mm³ L-1 and significant differences for the total APP biovolume were found in the following order: Selenga Delta > South, Centre > North (Fig. 16). Thus, the APP comprised only 11 % of the total phytoplankton biovolume in the North, but 61 % in the Selenga Delta (Fig. 16). Eukaryotic APP dominated in the South, whereas cyanobacterial APP dominated in the Centre, North and Selenga Delta (Tab. 2).

The median nano- and microphytoplankton biovolume was 0.4 mm³ L-1 (Fig. 16). No significant differences were found between individual regions, either based on biovolumes (Fig. 16) or cell numbers. Bacillariophyceae dominated the nano- and microphytoplankton assemblage all over the lake in both years with 70 – 90 % by volume (Tab. 2), while all other groups contributed less than 10 %.

In 2002, nano- and microphytoplankton species were investigated in greater detail than in 2001. Therefore, the results presented here will be limited to 2002. On average 53 % of the nano- and microphytoplankton biovolume resulted from endemic species (see Appendix C - Tab. 1 for endemic species). Between 9 and 14 nano- and microphyto-plankton species were identified at the different sites and depths. The mean reciprocal Simpson diversity index for the whole lake was 3.96 (Shannon-Wiener 1.92) ranging at individual sites from 2.1 to 3.6 (Tab. 2), but no significant differences or trends between the regions, stations and depths could be found for the diversity indices.

Fig. 16. Regional variability of temperature, Chla, carotenoids, and biovolumes; bars represent means with 95 % C.I. and boxes medians with interquartile ranges (25-75 %). A map of Lake Baikal showing the sampling sites is given for orientation. The APP biovolume vs. total phytoplankton biovolume ratios as well as the sum of carotenoids vs. Chla are given as percentages. The number of samples (n) for temperature and pigments was 43 - 52 in each of the three open basins and at Selenga Delta (Sdelta), 22 at Barguzin Bay (Bbay) and 6 at Academician Ridge (Aridge) and Maloe More (MM). The number of samples (n) for biovolumes was 9 - 12 in each of the three open basins and Selenga Delta.

All species were tentatively grouped into functional groups (Tab. 2) according to the scheme of Reynolds et al. (2002). The dominant functional group (by biovolume) was that of vernal bacillariophycean blooms typical of oligotrophic lakes (A), due to the dominance of Cyclotella species (Tab. 2). At a few sites the dominances changed. In the South (5 m) the dominant functional group was that usually found at the start of summer stratification in oligotrophic conditions (E) (Tab. 2). This group indicated slightly higher nutrient availability than the functional group (A). Also the groups (S1) and (Z) were found in the Selenga Delta, which indicated summer stratification (Tab. 2).

Tab. 2. Regional variation of diversity, functional groups and of the contribution to total biovolume. Only those species were listed here, which contributed more than 1 % at any of the sites.

Besides Cyclotella species, flagellata, most of them belonging to Chrysophyceae, were numerous across the whole of the lake (Tab. 2). The contribution to the total biovolume of these flagellates was nonetheless higher in the South and Selenga Delta than in the North (Tab. 2). A high number of Koliella longiseta (Chlorophyta) was found besides Cyclotella in the South and Centre, as well as a high number of Nitzschia acicularis in the Centre (Bacillariophyceae; Tab. 2). In the North Cryptomonas spp. (Cryptophyta) were found among the flagellates in high concentrations as well as Rhodomonas pusilla (Cryptophyta), which dominated the cryptophycean species at all other sites (Tab. 2). Furthermore, in the North the contribution of Cyclotella species to the total biovolume was higher than at all other sites unless Academician Ridge (Tab. 2). At Academician Ridge, Cyclotella cells were very numerous and the biovolume per Cyclotella cell was much higher than at the other sites. Their diameter reached 150 µm. The number of Aulosira sp. (cyanobacteria) cells was the highest at both sample sites within the large Selenga Delta, but they were much less abundant (> 5 times) at 5 m than at 10 m and 30 m (Tab. 2). Potential grazers of these filamentous cyanobacteria (the ciliophora Strombidium sp.) were numerous at 5 m but not in the deeper layers. The respective Aulosira species differed morphologically from A. implexa and A. laxa, which up until now were the only Aulosira species described for Lake Baikal (Bondarenko 1995) and has thus not yet been described for Lake Baikal. The description will be done elsewhere.

Regional differences of temperature and Chla: The mean July temperature across the whole lake (including all sampled depths) during the three expeditions was 8° C (Fig. 16, Fig. 17). A significant decline in temperature was found along the transect South > Centre > North (Fig. 16), with the difference between Centre and North only given at 90 % C.I. The Selenga Delta and Barguzin Bay had the highest temperatures (Fig. 16, Fig. 17). Nonetheless, the surface water (upper 5 m) in the Maloe More was also warmer compared to the open basins (Fig. 17).

The Chla concentration was significantly correlated to temperature. The whole lake average Chla concentration (2001-2003, including all sample depths) was 1.71 ± 0.19 nmol L-1 (95 % C.I., n=222; Fig. 16). The Chla concentrations in the three open basins (South, Centre and North) were significantly lower compared to those at the river inflows (Selenga and Barguzin) and within the open basins the North had a significantly lower Chla concentration than the South (Fig. 16). The warm surface waters of the shallow Maloe More were Chla-rich compared to the open basins (Fig. 17). The interpolation of the Chla results showed furthermore the transition from Chla-rich near-shore regions to Chla-poor open basin regions (Fig. 17).

Fig. 17. Map of Chla (left) and temperature (right) by nearest neighbouring interpolation from direct water samples taken from surface waters (0.5-5 m).

Fluorescence horizontal transects (c. 3 m water depth) performed in July 2002 and July 2003 during ship travel confirmed the interpolation of the Chla and temperature distribution (Fig. 18). From those transects the changes within the basins and towards the river inflows and near-shore regions could be continuously tracked and these transects confirmed the extremely heterogeneous Chla distribution and temperature gradients along the lake (Fig. 18). From the Ushkanin Islands (in front of the west coast of the Svyatoi Nos peninsula) along the west coast of Svyatoi Nos towards the Barguzin inflow, for example, the temperature increased from 10° to 15° C and the Chla concentrations doubled at the same time from 1.6 to 3.2 µmol L-1 (Fig. 18-1). From the North of Svyatoi Nos along the east coast of Olkhon Island the profiles indicated a clear shift from higher temperatures and Chla concentrations near Svyatoi Nos to low temperatures and Chla concentrations in the open water of the Central basin; and temperature and Chla concentration increased again nearing the southern end of Olkhon Island (Fig. 18-2). A profile from the harbour of Listvianka (South basin) to the North of the Selenga Delta showed furthermore the increase of temperature and Chla concentration nearing the Selenga Delta (Fig. 18-5).

Regional distribution of carotenoids: The total carotenoid concentration was significantly correlated to Chla (r²= 0.94, p< 0.001, n= 222) and was 85 % of the Chla concentration considering molar ratio (60 % considering weight ratios; Fig. 16). As was shown for the Chla variations, the total carotenoid concentrations were significantly lower in the open basins than at the river inflow sites and the North also showed significantly lower carotenoid concentrations than the South (Fig. 16). The ratios of carotenoids that collected light (such as fucoxanthin) to carotenoids that protected cells against high light (such as diadinoxanthin) was not correlated to the total carotenoid vs. Chla ratio (data not shown). Only at Maloe More the very low carotenoid vs. Chla ratio clearly resulted from lowering protecting carotenoids, indicating thus low light acclimation of the phytoplankton at that site.

Taking a lake average (July 2001, 2002 and 2003), the dominant carotenoids were zeaxanthin, fucoxanthin and lutein. Fucoxanthin (Fig. 19A), Chlc (Fig. 19A), diadinoxanthin and diatoxanthin concentrations showed no significant differences between the open basins, but significantly higher values were found at Barguzin Bay and at Academician Ridge. Chlb and lutein showed significantly higher concentrations in the South than in the Centre and North, as well as at Academician Ridge and Maloe More (Fig. 19B). Alloxanthin (Fig. 19C) and peridinin did not show significant variations between the open basins and the remaining regions. Violaxanthin concentrations showed significant decreases in the order South > Centre > North (Fig. 19C). Zeaxanthin and ß-carotene were significantly highest in the Selenga Delta, and the South showed significantly higher concentrations than the North (Fig. 19D).

A discriminant analysis (Wilk´s Lamba=0.08, p<0.001; Fig. 20) indicated, that considering the pigment composition, Academician Ridge and Barguzin Bay (root 1, p<0.001) as well as Maloe More (root 2, p<0.001) were most distinct from all other regions. The major pigment within root 1 was fucoxanthin (standardised canonical coefficient = 0.89) and the major pigment, influencing root 2, was violaxanthin (standardised canonical coefficient = -1.17), indicating major influence of Bacillario-phyceae+Chrysophyceae at Academician Ridge and Barguzin Bay and of Eustigmato-phyceae at Maloe More.

Fig. 20. Discriminance analysis separating the seven regions according to the pigment distri-bution. See Fig. 19 for abbreviations.

Estimation of phytoplankton composition using marker pigments: Accessory pigments can be used to estimate the composition of the phytoplankton assemblage (cf. Fietz and Nicklisch 2004, Appendix A). Based on the whole data set of 2001 to 2003 (n= 222) significant molar Chla/marker pigment ratios were calculated using multiple linear regression. This resulted in the following equation:

Chla = 1.26·Fuco + 1.62·Allo + 3.0·Chlb + 0.61·Zea` + 6.49·Viola` (Tab. 3A), whereby Chla was the total Chla from the five phytoplankton groups, “Fuco” (fucoxanthin) was the marker for Bacillariophyceae+Chrysophyceae, “Allo” (alloxanthin) was the marker for Cryptophyta, “Chlb” was the marker for Chlorophyta, “Zea´” was the marker for cyanobacterial APP and “Viola´” was the marker for Eustigmatophyceae. Zea´ was the cyanobacterial zeaxanthin and Viola´ was the eustigmatophycean violaxanthin only (cf. Fietz and Nicklisch 2004, Appendix A for calculations; cf. Fietz et al., submitted, Appendix B for having considered Eustigmatophyceae to be a common member of the Baikalian phytoplankton). The coefficient of determination (r²) was high (0.97; Tab. 3A) and the calculated Chla matched the measured Chla with a mean error of 6 % and a maximum error of 24 % in all regions and years.

The contributions to total Chla of the chemotaxonomic groups varied between the regions (Fig. 21). Whereas Bacillariophyceae+Chrysophyceae contributed only 20 % to the total Chla in the South and Selenga Delta, its contribution was higher (45-50 %) in the North and Barguzin Bay and highest (70 %) at Academician Ridge. The contribution of Cryptophyta was small (< 15 %) all over the lake. The contribution of Chlorophyta to the total Chla was highest in the South (40 %) and lowest at Academician Ridge and Maloe More (< 10 %). The contribution of Eustigmatophyceae was about 20 % in the South and Selenga Delta and was highest at Maloe More (45 %). Possibly several Chrysophyceae contributed to the total violaxanthin at Maloe More so that the eustigmatophycean violaxanthin was overestimated at that site. The contribution of the cyanobacterial APP was highest in the Selenga Delta (30 %) and lowest in the North, Academician Ridge and Maloe More (<5 %).

Tab. 3. Molar ratios of Chla vs. marker pigments and their respective statistics calculated by multiple linear regression (A) for the complete data set (July 2001, July 2002 and July 2003) and (B) regionally differentiated.

A) Whole lake

molar ratio

P

partial c.

95 % C.I.

P

r²

Chla / Fucoxanthin

1.26

< 0.001

0.81

0.12

< 0.001

0.98

Chla / Chlb

3.00

< 0.001

0.69

0.42

Chla / Alloxanthin***

1.62

< 0.001

0.37

0.54

Chla / Zeaxanthin*

0.61

< 0.001

0.82

0.06

Chla / Violaxanthin**

6.49

< 0.001

0.84

0.57

B) Regions

molar ratio

P

partial c.

95 % C.I.

P

r²

South

Chla / Fucoxanthin

1.17

< 0.001

0.77

0.33

< 0.001

0.99

Chla / Chlb

3.28

< 0.001

0.95

0.37

Chla / Alloxanthin***

1.55

< 0.05

0.33

1.50

Chla / Zeaxanthin*

0.79

< 0.001

0.96

0.07

Chla / Violaxanthin**

1.64

<0.005

0.45

1.10

Centre

Chla / Fucoxanthin

1.57

< 0.001

0.94

0.18

< 0.001

0.99

Chla / Chlb

3.18

< 0.001

0.87

0.57

Chla / Alloxanthin***

1.17

< 0.001

0.55

0.57

Chla / Zeaxanthin*

0.57

< 0.001

0.77

0.15

Chla / Violaxanthin**

3.35

< 0.001

0.52

1.76

North

Chla / Fucoxanthin

1.12

< 0.001

0.64

0.38

< 0.001

0.94

Chla / Chlb

7.36

< 0.001

0.73

1.96

Selenga Delta

Chla / Fucoxanthin

1.39

< 0.001

0.69

0.45

< 0.001

0.98

Chla / Chlb

3.87

< 0.001

0.70

1.24

Chla / Alloxanthin***

1.48

0.08

0.26

1.69

Chla / Zeaxanthin*

0.66

< 0.001

0.86

0.12

Chla / Violaxanthin**

3.45

< 0.05

0.32

3.16

Barguzin Bay

Chla / Fucoxanthin

0.79

< 0.001

0.68

0.41

< 0.001

0.97

Chla / Chlb

5.77

< 0.001

0.85

1.71

Chla / Violaxanthin**

9.75

< 0.001

0.79

3.61

* cyanobacterial zeaxanthin only (see text)** eustigmatophycean violaxanthin only (see text; see Fietz et al., submitted, Appendix B for Eustigmatophyceae)*** In a preliminary study (Fietz and Nicklisch 2004, Appendix A) it was mentioned that caloxanthin, a zeaxanthin transformation product, possibly coeluted with alloxanthin in some samples. In the present study, the correlation between α-carotene and alloxanthin, both contained in Cryptophyta, was significant. Therefore we assumed that alloxanthin in almost all samples did not coelute with caloxanthin. Few outliers from the significant alloxanthin/α-carotene relationships were omitted in the multiple linear regression calculations of the present study.

The factors calculated for the whole lake do not account for the variances of differential phytoplankton compositions in the individual regions of the lake (as indicated by the regional variations of the species and marker pigments, Fig. 19). Therefore, individual Chla/marker pigment ratios were calculated for each region (Tab. 3B). The splitting of the data set, however, limited the calculation of significant factors in the North and also in Barguzin Bay (Tab. 3B). In most cases the regionally differentiated factors did not differ significantly from those calculated for the whole lake data set (Tab. 3A,B) and, therefore, the use of the whole lake factors can be recommended.

Interannual variability: Significant differences in the Chla concentrations were found in each basin when the three years were compared, particularly in the North (Fig. 22A). In 2001 the Centre had significantly the lowest Chla concentrations, while in 2002 concentrations were lowest in the North (Fig. 22A). Similarly, significant interannual differences were found for each of the carotenoids, for Chlb (Fig. 22B), for Chlc (Fig. 22C), for the sum of carotenoids and for the carotenoids vs. Chla ratios (data not shown), particularly in the North and Centre.

High interannual variability was also found for the cyanobacterial and eukaryotic APP biovolume, particularly in the South and Centre ( Fig. 23A). Nevertheless, the North had lowest APP biovolumes in both years and the Selenga Delta the highest (Fig. 23A). Additionally, high interannual variability was found for the nano- and microphytoplankton biovolume, particularly in the North, where a much lower biovolume was determined in July 2002 compared to July 2001 (Fig. 23B).↓44 This change in biovolume was mainly due to shifts in the species composition that means shifts in the biovolume/cell ratio (mainly of Bacillariophyceae, data not shown). As has been shown for the biovolumes (Fig. 23A,B) and for chlorophylls and carotenoids (Fig. 22), the estimated contributions of the distinct chemotaxonomic groups differed also between the three investigated years (Fig. 23C).

Temperature and chlorophyll depth profiles from January 2001 to December 2003 near Bolshye Koti (see Fig. 16 for location) are plotted in Fig. 24. A thin convective layer occurred under the ice in February, where temperatures below 0.5° C were found down to 25 m (Fig. 24). The temperature increased to 3° C between 50 m and 100 m, and remained between 3 and 4° C from 250 m down to the lake bottom (Fig. 24). Therefore, this period showed an inverse stratification, with mixing restricted to a shallow layer under the ice. Every year wind-induced mixing spanning over the upper 250 m occurred from May to June, after ice-break up, and another in November (Fig. 24). During spring overturn in 2001 Chla concentrations up to 0.5 nmol L-1 were found down to 100 m and up to 0.25 nmol L-1 down to 250 m (Fig. 24). During the spring overturn in 2002 Chla concentrations over 2 nmol L-1 were found down to 100 m and up to 0.5 nmol L-1 down to 250 m, while during summer stratification the Chla concentration was low (<0.2 nmol L-1) below 50 m (Fig. 24).

Fig. 24. Temperature and Chla depth profiles (0-250 m) from January 2001 to December 2003 showing mixing and stratification over the season and regions. Values at the time of ice break-up (April) have been interpolated. From Mid-April to end of August 2003 data below 50 m were not available (hatched rectangle).

Fig. 25. Vertical variability of temperature, pigments (see Fig. 16 for bars and boxes). The APP biovolume vs. total phytoplankton biovolume ratios as well as the sum of carotenoids vs. Chla are given as percentages. The number of samples (n) for temperature and pigments was 21 at 0.5 m, 110 at 5 m, 43 at 10-20 m, 36 at 20-30 m and 6 at 45-85 m. The number of samples (n) for biovolumes was 10 - 18 at each depth.

The average July temperatures of the lake (2001-2003) decreased significantly with the depth in the order: 0.5 m, 5m > 10-20 m > 20 – 30 m, 45 – 85 m (Fig. 25). Stratification and mixing might also result from conductivity in oceanic and also some freshwater systems, but in Lake Baikal conductivity varied only between 100 to 110 µS cm-1 (corrected to 20° C) between the individual regions as well as with the depth (Fig. 26). Mixing conditions varied among the different basins and regions. High-resolution temperature and conductivity depth profiles showed that in the North mixed conditions prevailed, while a weak stratification developed in the South and Selenga Delta. Stable stratification with a broad epilimnion developed in Barguzin Bay (Fig. 26). There, the conductivity showed a second maximum at 20-25 m, probably indicating the influence of subsurface water currents induced by the river (Fig. 26). The Chla concentration in July showed a maximum at 16 m in the South, whereas it was rather homogeneously distributed in the North and Centre. It decreased with depth in the Barguzin Bay and most prominently in the Maloe More (Fig. 26).

The combined data set of all samples collected during the July expeditions 2001 and 2003 revealed a significant decrease of the Chla concentration and the sum of carotenoids below 20 m compared to the 5 m and 10 – 20 m samples (Fig. 25). The percentage sum of carotenoids compared to the Chla was also lowest (74 % molar ratio) in the 20 – 30 m layers (Fig. 25). At several stations, deep Chla maxima were found in depths of 45 m, 50 m and 85 m. Within these deep maxima the Chla concentrations were as high as above 20 m (Fig. 25). The sum of carotenoids was lower in these deep layers than above 20 m and the percentage molar ratio was only 69 % (Fig. 25). At 45 – 85 m the ratio of ´light-collecting´ vs. ´protecting´ carotenoids was highest (data not shown), indicating that the lower total carotenoid vs. Chla ratio in greater depths resulted from reduced protecting carotenoids. Thus, as expected low-light conditions prevailed at the great depths, but high light conditions down to 10 – 20 m.

The total APP also decreased in the 20 – 30 m samples compared to the 5 m samples, whereas no significant changes could be found for the nano- and microphytoplankton (Fig. 25). There were some tendencies in both years that biovolumes differed with depths within the individual regions. The relatively shallow Selenga Delta for example showed a maximum of the APP biovolume at 16 m depth and of the nano- and microphytoplankton at 5 m (Tab. 2). Several species showed also distinct relationships with depth. Aulacoseira baicalensis for example formed a deep maximum in 85 m water depth in the South (Tab. 2).

Tab. 4. Canonical correlation analysis of set one comprising the environmental variables, and set two comprising total calculated Chla and the respective percentage contribution of each phytoplankton group to the total calculated Chla. See text for calculation of the contributions.

Regional distribution

Seasonal distrib.

Canonical loadings

Canon. loadings

Root 1

Root 2

Root 3

Root 4

Root 1

Root 2

Set 1, environmental variables

Latitude

-0.52

-0.67*

0.10

0.52

-

-

Water Depth

-0.51

0.44

-0.71*

0.20

-

-

Stratification

0.99*

-0.05

-0.15

-0.05

-0.73*

-0.68

Temperature

0.53

-0.47

-0.20

-0.68*

-0.74*

0.67

Set 2, phytoplankton group contribution

Total Chla

0.87*

-0.40

-0.01

0.08

-0.51

-0.57*

Bacillario.+Chrysophyceae

-0.28

-0.44

0.74*

0.16

0.39

-0.57*

Chlorophyta

-0.06

0.92*

0.09

0.14

0.44*

-0.22

Cyanobacterial APP

0.46*

-0.02

-0.29

-0.07

-0.91*

0.07

Eustimatophyceae

0.16

-0.12

-0.30

-0.84*

-0.33

0.38*

Cryptophyta

-0.62*

-0.10

-0.50

-0.02

0.16*

-0.04

Root 1

Root 2

Root 3

Root 4

Root 1

Root 2

Eigenvalue

0.51

0.27

0.10

0.03

0.75

0.20

Canonical Correlation (r)

0.72

0.52

0.31

0.18

0.86

0.45

Significance (p)

0.00

0.00

0.00

0.06

0.00

0.30

Percentage variance, set 1

0.45

0.22

0.14

0.19

0.54

0.46

Percentage variance, set 2

0.24

0.20

0.16

0.13

0.26

0.14

Redundancy, set 1

0.23

0.06

0.01

0.01

0.40

0.09

Redundancy, set 2

0.13

0.05

0.02

0.00

0.19

0.03

* Highest values within a set for each canonical variate.Note: The stratification was determined as 0=mixed and 1=stratified according to the temperature profiles carried out with the CTD-probe and/or submersible FluoroProbe.

Driving factors for phytoplankton distribution: Canonical correlation analysis (CCA; Tab. 4) indicated that total Chla as well as the percentage contributions of cyanobacterial APP and Cryptophyta to the total Chla were correlated with the first canonical variate, which explained 24.5 % of the variance. The percentage contribution of Cryptophyta to the total Chla was negatively correlated to the first canonical variate. Considering the environmental data the first canonical variate showed the strongest correlation to the stratification (Tab. 4). The second canonical variate was related to the contribution of Chlorophyta and to latitude. The third canonical variate was related to the contribution of Bacillariophyceae+ Chrysophyceae and to the water depth. The fourth variate was not significant. Although CCA does not provide direct covariation between the two sets of variables, we may state for example, that cyanobacterial APP showed highest contributions where the water column was stratified, whereby the temperature was secondary. Chlorophyta dominated in the low latitude regions, probably related to the insolation, as temperature was not of great importance. Bacillariophyceae+ Chrysophyceae dominated, where the water depth was high, that means rather in the open basins than in the near-shore or river inflow regions.

Fluorescence depth profiles: As has been suggested from the CCA, mixing and stratification were very important factors for the differential phytoplankton group distribution. Thus, in situ fluorescence depth profiles, showing the mixing/stratification regime of the water body by recording the temperature and the total Chla were analysed in July 2002 and July 2003 (Fig. 27, Fig. 28) and August 2002 (Fig. 29). Basically these profiles strengthened two trends: The first one was a trend from near-shore to offshore, well visible for example from the Barguzin transect where the regime changed from a stratified one to a mixed one with the increasing distance from the shore towards the open Central basin (Fig. 27). Another trend in July was the one from the South to the North. While in the South the water body was almost always stratified, the water body of the North was mixed (Fig. 28). Nonetheless, this trend was much less expressed in August. Then, almost all stations were stratified, even those of the North (Fig. 29).

The August depth profiles exhibited great heterogeneity for the Chla maxima even between neighbouring sites, which in Lake Baikal are, however, still a few kilometres away from each other (Fig. 29). In the South, the epilimnion spanned up to 50 m and the Chla maxima mostly occurred up to that depth (Fig. 29). Only in the central part of the South basin, where water depth reached up to 1400 m, the epilimnion was limited to 20 m (Fig. 29). Then, the Chla maximum was found between 10 and 20 m within the thermocline and the maximum Chla concentration was much higher than at all the near-shore stations. In the North several Chla maxima were also found in depths varying from 10 to 50 m (Fig. 29). The Chla maxima occurred often within the thermocline, as has been shown for the deep-water stations in the South (Fig. 29). Some of these trends were already indicated by the direct water samples (Fig. 26), but using continuous measurements, the resolution was strongly increased. Using direct samples, the risk persist to overlook deep Chla maxima.

However, it is worth to be mentioned here, that these data are uncorrected for humic substances and not calibrated including all dominant phytoplankton groups. Nonetheless, preliminary correlation analyses of A. Nicklisch (HU Berlin, Germany, pers. comm.) indicated that total Chla concentrations measured with the FluoroProbe fitted well to the total Chla determined by HPLC (r² = 0.96).

Fig. 27. Chla and temperature depth profiles assessed with the submersible fluorimeter in July 2002. Transect from near-shore to off-shore (from Barguzin Bay to eastern shore of Olkhon Island).

Fig. 28. South to North transsects of Chla and temperature depth profiles in July 2002 (red circles, profiles on left side) and July 2003 (yellow circles, profiles on right side).

Fig. 29. Depth profiles of Chla (green lines) and temperature (blue lines) in August 2002. See Fig. 28 for legend, whereby in August maximum measured depth was 100 m and maximum temperatures were 18°, 16° and 14° C in the South (including Selenga Delta), Centre and North, respectively.

Intense monitoring was conducted from May 2002 to June 2003. Variations from January 2001 to December 2003 are plo than 2001 and 2003 with a mean summer (July-September) temperature at 5 m water depth of 12° C, compared to 8.5° C in 2001 and 8° C in 2003. Peak temperatures reached 17.4° C in 2002 compared to 15.4° C in 2001 and 11.6° C in 2003. The ice cover lasted shortest in 2002 (1 month) relative to 2001 (2.5 months) and 2003 (2 months). In every year the integrated Chla below the ice was as high as in summer. Generally four peaks could be discerned (around March, June, July and September), but their intensities varied interannually. The spring peak 2001 was probably missed due to the inaccessibility of the sampling station. The spring peak 2002 was by far the highest within the three investigated years.

Seasonal dynamics of APP, nano- and microphytoplankton: During the intense monitoring 2002-2003, only spot samples could be counted with the epifluorescence microscope for APP. The amount of APP was high in July (about 0.7 mm3 L-1) and decreased towards autumn (less than 0.2 mm3 L-1; Fig. 30B). The APP started to increase again by the end of March (Fig. 30B). Nevertheless, the presence of APP in February and March (about 0.1 mm3 L-1), when the lake was covered with ice, indicated an important APP formation under the ice. Light microscopic estimations of the total APP during the whole season supported the conclusions drawn from epifluorescence spot checks (Fig. 30B).

The median annual nano- and microphytoplankton biovolume was 0.07 mm3 L-1. In spring the nano- and microphytoplankton biovolume and the cell number were high, while the diversity was low (Fig. 30B). This spring bloom (2002) was dominated by Stephanodiscus meyerii and Aulacoseira baicalensis (Bacillariophyceae) that belong to the indicators of vernal blooms usually found in mixed, mesotrophic conditions (B; Tab. 5). At the end of May to the beginning of June the nano- and microphytoplankton biovolume and cell number decreased, while the diversity increased to a maximum in Mid-June (Fig. 30B). The diversity was, therefore, highest at the time of the minimum biovolume and cell abundance.

The nano- and microphytoplankton biovolume was then low during summer and autumn, but the cell number, in contrast, was in summer as high as during spring (Fig. 30B). Towards the beginning of July (2002), the contribution of Bacillariophyceae decreased and Chlorophyta (Koliella sp. and Monoraphidium sp.) became important (Fig. 30B; Tab. 5). The dominant functional groups changed to mixed, oligotrophic (E) or mesotrophic conditions (X2; Tab. 5). In July the assemblage comprised mainly Koliella longiseta (Chlorophyta) and Rhodomonaspusilla (Cryptophyta) (Tab. 5). According to the functional groups the large number of chrysophycean flagellates indicated the start of summer stratification (E) (Tab. 5). Bacillariophyceae were rather rare at that time of the year (Fig. 30B; Tab. 5). Small algae such as R. pusilla and Chrysophyceae (chrysophycean flagellates as well as Chrysidalis sp. and Dinobryon spp.) predominated also during the whole August and September (Tab. 5). Asterionella formosa (Bacillariophyceae) reached a mass development (10-fold increase within one week) in autumn that was unusual for the Bolshye Koti site (Tab. 5).

Under the ice of the next year (2003) the biovolume and cell number were low, but the diversity was as high as in summer (Fig. 30B). The assemblage was dominated by species preferring cold, mixed and enriched conditions (C), such as A. formosa (Bacillariophyceae) (Tab. 5). Shortly before ice-break-up Synedra acus (Bacillariophyceae) and Gymnodinium baicalensis (Pyrrophyta) became dominant (Tab. 5). The former prevail in vernal mixed conditions (D), but, in contrast to larger Bacillariophyceae, such as Cyclotella, they might not be sensitive to stratification. Gymnodinium species are mostly sensitive to mixing and dominated in stratified epilimnia (LM; Tab. 5). Both groups indicate conditions of nutrient availability. The bacillariophycean maximum expected after ice-break up 2003 was probably missed, because sampling had to be stopped during ice-break up until a time when conditions permitted ship access to the sampling location.

Seasonal dynamics of phytoplankton pigments: The biovolume maximum did not correspond to the chlorophyll and carotenoid one (Fig. 30B). Shifts towards Chla-rich phytoplankton groups prevailed in summer, while large and Chla-poor cells dominated in winter and spring. Thus, the Chla vs. nano- and microphytoplankton biovolume ratios were < 6 nmol mm-3 in spring (May-June) but 22 nmol mm-3 on average in summer (July-September) and 8.5 nmol mm-3 on average under the ice (February-April).

After the ice break-up at the end of April 2002, Chla showed a first maximum in Mid-May (2 nmol L-1), decreasing strongly from the end of May to the beginning of June (0.9 nmol L-1; Fig. 30B). The sum of carotenoids was about 90 % of Chla (based on molar ratios and 60 % based on weight ratios) during the time of spring mixing. Secchi depth was low (10 m) at the Chla maximum but increased while chlorophylls and carotenoids decreased (Fig. 30B).

Fig. 30. Seasonal monitoring during May 2002 to June 2003 (at the long-term monitoring station of SRIB 2.8 offshore from Bolshye Koti, South Basin) of temperature, Secchi depth, biovolumes and selected pigments.(A) temperature at 5 m water depth and Chla integrated over 40 m water depth (the suggested euphotic zone) from January 2001 to December 2003. The highlighted area designate the period of the intensive monitoring, detailed in section (B): Intensive monitoring from May 2002 to June 2003. On the left side: Secchi depth, temperature, total APP biovolume (epifluorescence microscopic spot counts + light microscopic estimation), total nano- and microphytoplankton (NMP biovolume and cell abundance), reciprocal Shannon-Wiener diversity index, and composition of the phytoplankton community based on their share to total Chla (see text for calculations). On the right side: total Chla as well as marker pigments and biovolumes of the respective phytoplankton groups.

The clear-water phase gave way to another Chla maximum in Mid-July (2 nmol L-1; Fig. 30B) and the sum of carotenoids increased up to 160 % of Chla. The Chla concentration was high during summer, when temperatures rose to a maximum of 16° C and Secchi depth was less than 10 m (Fig. 30B). The carotenoids were about 100 % of Chla during the summer stratification. From the end of summer to autumn the temperature and Chla concentration and percentage carotenoids decreased while the Secchi depth increased (Fig. 30B). From February to March 2003 the Chla concentration increased under the ice and Secchi depth was decreased (Fig. 30B). The sum of carotenoids was low (70 % of Chla) under the ice.

The distinct pigment changes likely reflected the variations in the phytoplankton groups. Similar to the biovolume of the Bacillariophyceae, all related pigments, such as fucoxanthin, Chlc and diadinoxanthin showed maxima in May after ice-break up (Fig. 30B). In summer, Bacillariophyceae biovolume decreased while the related pigments decreased to a lesser extent, marking shifts towards smaller, pigment-rich Bacillariophyceae or towards Chrysophyceae (which also contain those pigments). Microscopic counts confirmed the shift towards Chrysophyceae (flagellates, Tab. 5).

Chlorophyta and its related pigments also showed some discrepancies (Fig. 30B). While the Chlorophyta biovolume was low in spring 2002 and 2003, lutein and Chlb were high (Fig. 30B). In summer, both pigments increased faster than the biovolume, suggesting the influence of chlorophycean APP with high pigment vs. biovolume ratios (Fig. 30B). The summer maxima and the winter decrease of zeaxanthin and ß-carotene correspond to the course of the cyanobacterial APP (Fig. 30B). Except cyanobacterial ones, pigment concentrations were also high under the ice and pigments of Cryptophyta (alloxanthin) and Pyrrophyta (peridinin) even showed maxima.

Estimation of phytoplankton composition using marker pigments: As has been shown for the regional variability, the contributions of distinct chemotaxonomic groups to the total Chla can be calculated using the marker pigments. Based on the factors listed in Tab. 3A for the whole lake average, contributions have been calculated for the period of the intense monitoring (Fig. 30B): Bacillariophyceae+Chrysophyceae had highest (up to 90 %) proportions of total Chla in spring, autumn and winter, Chlorophyta and Eustigmatophyceae in early summer, and cyanobacterial APP in summer; Cryptophyta contributed low proportions to total Chla throughout the year.

The calculated Chla matched the measured one with a median error of 2 % in spring (May-June), of 13 % in summer (July-September), but of 38 % in fall (October-December) and of 31 % under the ice (February-March). It matched the measured Chla in June 2003 again with an error of only 5 %. As the applicability of the factors differ with the phytoplankton composition, the regional factors, accounting for these heterogeneities (Tab. 3B), were also tested. The best fit for the summer community was reached using the Selenga Delta factors (Tab. 3B) with a median error of only 10 % between calculated and measured Chla. Best fits for autumn (Selenga Delta factor) and under ice (Centre factor) communities, in contrast, implied errors greater than 25 % and thus, these communities were different from all others.

Driving factors for seasonal succession: Canonical correlation analysis (CCA) showed that stratification and temperature dominated the first canonical variate (Tab. 4). The percentage contribution of Chlorophyta and cyanobacterial APP to total Chla were highly correlated to this first variate, as well as, although negatively, Cryptophyta. The second variate was not significant. Taken together the CCA for regional and seasonal variations indicated that temperature and stratification were of major importance for the phytoplankton development in Lake Baikal and potentially influence predominantly small cells such as cyanobacterial picoplankton.

Through the upper water column (water samples), the deeper water column (sediment traps) and also within the water to sediment interface (top sediment slice) major lipophilic photosynthetic pigments known from freshwater samples were detected by the HPLC-aided analysis (Tab. 6). Characteristic fluorescence and absorption chromatograms are shown in Fig. 31 for the water samples (Fig. 31A), the 40 m trap (Fig. 31B), the 1400 m trap (Fig. 31C) and the top sediment slice (Fig. 31D).

Tab. 6. Major lipophilic photosynthetic pigment fluxes into the 40 m trap and into the trap at the lake bottom. Values for the lake bottom (1400 m) were extrapolated from the curve fittings shown in Fig. 32 and Fig. 33 for all pigments with exponential decrease. For pheophytin a, pyropheophytin a and pheophytin b, which did not show a significant decrease with depth, 40 m and lake bottom values were calculated as averages of all traps. Chla included allo-, epimers and other derivates; pheophorbide a included all pheophorbide a derivates and pheophytin a all pheophytin a derivates.

Pheopigments, degradation products of chlorophylls, were found only in sedimented material (Fig. 31, Tab. 6). Four different pheophorpide a -like pigments were found (Fig. 31). The occurrence of those pheophorpide and pyropheophorbide derivates was not correlated with the depth. Thus, all of them are grouped as “pheophorpide a” in the following text. All pheophytin a -like pigments were also grouped to “pheophytin a” in the text below. Pyropheophytin a could be clearly differentiated from pheophytin a and is discussed separately (Fig. 31, Tab. 6). The chromatograms of pigment extracts of the sediment traps and sediment slices also contained several unidentifiable components (unnumbered peaks in Fig. 31). They were strongly degraded pigments, which were not included in our quantitative comparisons.

Transfer fluxes: During 16 months of deployment 239 g m-2 dry matter (DM) settled in the 40 m trap, with an average flux of 14.9 g m-2 month-1 (Tab. 7, Fig. 32). The content of total organic carbon (TOC) was 21.9 % at that depth and that of total nitrogen (TN) 1.6 % (Tab. 7, Fig. 32). The resulting atomic ratio of TOC vs. TN (C/N) of 15 indicated that the sedimented material resulted from the autochthonous production by suspended phytoplankton and that terrigenous input is likely to be negligible at that site. The amount of pigments gathered during the 16 months deployment in the 40 m trap was 193.1 µmol m-2 for Chla and 797 µmol m-2 for chlorophyllide a+pheopigment a (Tab. 6). It is worth noting that the replicate samples of the 40 m trap deviated strongly (coefficient of variation: 60.5%), whereas the coefficients of variation for the replicate samples in the traps below varied from 2.5% to 15.5%. Pheophorbide a was the most prominent degradation product in the 40 m trap (Tab. 6, Fig. 32). With respect to TOC (total organic carbon) only 3.9 µmol g-1 Chla but 9.47 µmol g-1 pheophorbide a were found (Tab. 8). The lowest ratio was found for pyropheophytin a/ TOC (Tab. 8).

Tab. 7. Sedimentation and accumula-tion rates of the dry matter, total nitrogen and atomic C/N ratio in the 40 m trap and at the lake bottom. Values for the lake bottom (1400 m) were extrapolated from the curve fittings shown in Fig. 32 (see also Appendix C - Tab. 2. )

40 m trap

1400 m trap

dry weight (g m-2 month-1)

14.9

9.49

TOC (%)

21.87

7.14

TN (%)

1.60

0.76

C/N (mol mol-1)

14.80

9.91

* Müller et al. 2005

Tab. 8. Ratios of Chla and its degradation products per organic carbon in the 40 m trap and at the lake bottom. Values for the lake bottom were extrapolated from the curve fittings (Fig. 34, Appendix C - Tab. 4.). Data for chlorophyllide a/ TOC and pheophorbide a/ TOC, which did not show significant depth trends, were calculated as averages of all traps

40 m trap

1400 m trap

µmol g-1

chlorophyll a/ TOC

4.1

4.38

chlorophyllide a/ TOC

1.14

pheopheorbide a/ TOC

9.87

pheophytin a/ TOC

2.25

8.12

pyropheophytin a/ TOC

0.08

0.42

In the 40 m trap fucoxanthin was the dominant carotenoid (Tab. 6, Fig. 33). Other pigments of Bacillariophyceae+Chrysophyceae (Chlc, diadinoxanthin and diatoxanthin) as well as the cyanobacterial zeaxanthin also showed high sedimentation rates, whereas the chlorophyte Chlb and lutein as well as the cryptophyte alloxanthin, sedimented only in low amounts (Tab. 6, Fig. 33).

A principal component analysis (PCA) including the dry matter, TOC and TN as well as all pigments revealed that three components controlled 90.7 % of the variance. The first component (65.5 %) included the depth and controlled dry matter, TOC and TN. The first component controlled also the labile pigments Chla, b and c as well as chlorophyllide a and pheophorbide a and also all carotenoids. The second and third components comprised the more stable pigments pheophytin a, pyropheophytin a and pheophytin b.

The sedimentation to the lake bottom showed a power regression for dry matter, but two-exponential or two first order independent decay regressions were apparent for TOC, TN, Chla, b, c, chlorophyllide a, pheophorbide a and most carotenoids (Fig. 32, Fig. 33, Appendix C - Tab. 2, Appendix C - Tab. 3). In contrast, for the more stable chlorophyll degradation products, pheophytin a, pyropheophytin a and pheophytin b, none of those regression models fitted accurately (Fig. 32). The composite character of the regressions (Appendix C - Tab. 2, Appendix C - Tab. 3) indicated that the degradation passed through two different phases, triggered by different factors.

The first degradation phase occurred within the upper 250 m and was much stronger than the second. Below 250 – 300 m the degradation became visibly lowered. Calculations of simple exponential or decay models for those pigments resulted in much lower coefficients of determinations or insignificant models. Therefore, one simple phase cannot describe the pigment degradation with depth. The functions (Appendix C - Tab. 2 and Appendix C - Tab. 3) should allow reconstructions of initially settled pigments from trap or sediment data in further investigations. The models are nevertheless preliminary, as they do not take into account different degradation extents within the traps.

Different degradation patterns were revealed when chlorophylls and its degradation products were referred to TOC (Fig. 34) instead of area references (Fig. 32). The Chla/ TOC ratio decreased with depth, indicating that organic carbon is more slowly degraded than Chla (Fig. 34, Appendix C - Tab. 4), whereas the pheophytin a/ TOC ratio and the pyropheophytin a/ TOC ratio increased with the depth, indicating the formation of pheophytin and pyropheophytin with depth (Fig. 34, Appendix C - Tab. 4). Best fits for the chlorophyllide a/ TOC ratio and pheophorbide a/ TOC ratio vs. depth were also linear regression models, but they were not significant (Fig. 34).

Considering that the 40 m trap was collecting 100 % of the particles that settled out of the euphotic zone, 100 % dry matter would be 14.9 mg m-2 month-1. At 100 m 10 mg m-2 month-1 dry matter were collected which accounts for 67 %. At the lake bottom an estimate of 64 % of dry matter settled down. The estimated percentages of TOC and TN that reached the lake bottom were, in contrast, only 33 % and 48 %, respectively (Tab. 7, Fig. 32). The organic compounds were obviously more strongly degraded than the non-organic fraction (mainly siliceous valves of diatoms) of the settled material. The loss during the sedimentation was, however, even stronger for most of the pigments. Only 24 % of Chla reached the lake bottom and 21 % of Chlb (Tab. 6, Fig. 33). The distinct degradation products of Chla showed different losses. About 100 % of the pheophytin a reached the lake bottom, but only 16 % of pheophorbide a.

Composition of the settling material: Fucoxanthin, Chlb and zeaxanthin were used to estimate the contribution of Bacillariophyceae+Chrysophyceae, Chlorophyta and cyanobacterial picoplankton to total Chla. According to the factors established in Fietz and Nicklisch (2004, Appendix A-Table 3), the Chla content in the 40 m trap was composed of 87 % Bacillariophyceae+Chrysophyceae, 11 % Chlorophyta and 2 % cyanobacterial picoplankton (Fig. 35). The percentage contribution did not change with the water depth, as the same composition was found in the deepest traps (Fig. 35).

In all traps the calculated Chla concentration – based on the marker pigments – was much higher compared to the measured Chla concentration. On average the calculated value of the Chla concentration was 157 % of the measured value. Pheopigment a, which results from grazing and photooxidation, were not added to Chla because carotenoid degradation products, resulting from the same processes, were also not added to the marker pigments. Adding chlorophyllide a that occurs in senescent cells due to enzymatic lyses, which does not affect carotenoids (Jeffrey et al. 1997), the average calculated Chla+chlorophyllide a concentration was 121 % the measured value. This overestimation could result from an unusual high carotenoid or Chlb/Chla ratio of specific settling taxa or from a higher degradation rate of Chla than carotenoids or Chlb. The second assumption is likely.

A conclusion concerning the source of pheopigment a, as has been shown for Chla, is limited, because marker pigments which underwent a similar degradation then pheopigments are missed. The only share which can be calculated is for Chlorophyta assuming that the ratio of pheophytin a to pheophytin b represents the former ratio of Chla to Chlb in the settling material. Then, the factor for the Chla/Chlb relationship (cf. Fietz and Nicklisch 2004, Appendix A-Table 3), can be adopted for the pheophytin a/ pheophytin b relationship. According to this approach 2.6 % of pheophytin a should originate from Chlorophyta.

Fluxes and ratios in the top traps: Two mooring strings, containing 9 traps in the North and 15 in the South, were deployed during two ensuing periods (2001-2002 and 2002-2003). Annual fluxes to the uppermost trap varied strongly between both investigated deployment periods (2001-2002 and 2002-2003) and between the two investigated sites (South and North; Tab. 9A). At both sites (South and North) the Chlas (Chla + chlorophyllide a + pheopigment a) flux was much lower during 2002-2003 than during 2001-2002 (Tab. 9A). The dry matter (DM) and TOC fluxes were lower during the second period as well (Tab. 9A). The Chlas and TOC fluxes as well as the Chlas/TOC ratio were higher in the South than in the North in 2001-2002, but lower in 2002-2003 (Tab. 9A).

The fluxes of Chlbs, Chlc, fucoxanthin (Tab. 9A) and other carotenoids were lower during the second deployment period in both basins similar to the Chlas flux (whereby for the North the 300 m trap has to be considered when comparing both years, because the 50 m trap was lost during the first deployment). The Chlbs/Chlas ratio was much lower (1-3 %), than the Chlc/Chlas and fucoxanthin/Chlas ratios (42-48 % and 18.5-20 %, respectively) in the South and similar results were found for the North (cf. Tab. 9A). These ratios indicated a much greater proportion of the Bacillariophyceae+ Chrysophyceae (indicated by Chlc and fucoxanthin) flux into the top traps than of Chlorophyta (indicated by Chlb) in both years and at both sites.

Fluxes and ratios in the bottom traps: At both sites the Chlas fluxes were significantly lower during the second deployment than during the first (Tab. 9B). Furthermore, the Chlas fluxes, which reached the lake bottom, were significantly higher in the South than in the North. Compared to the top traps, the Chlas fluxes which reached the lake bottom were 80-95 % lower for the South mooring 2001-2002, and both North moorings, but higher in the South mooring 2002-2003 (cf. Tab. 9A,B).

The fluxes of Chlbs, Chlc, fucoxanthin (Tab. 9B) and other carotenoids as well as of dry matter and TOC (Tab. 9B) were (similar to the Chlas fluxes) also significantly higher in the South than in the North. No significant differences were found for the C/N ratios (Tab. 9A,B).

Tab. 9. Fluxes and ratios of dry matter, organic carbon and Chlas in (A) the top traps and (B) the bottom traps of the four investigated moorings, and (C) in the top of cores from below the moorings.

site

South

North

period

2001-2002

2002-2003

2001-2002

2002-2003

A) top traps

at depth

40 m

40 m

300 m

50 m (300 m)

DM

(g m-2 yr-1)

174.5

39.9

127.4

30.7 (19.7)

TOC

(g m-2 yr-1)

38.2

5.0

10.0

9.0 (2.5)

C/N

atomic ratio

14.8

12.4

11.8

9.9 (12.3)

Chlas

(µmol m-2 yr-1)

752.0

43.8

179.6

146.3 (70.8)

Chlas/DM

(µmol g-1)

4.3

1.1

1.4

4.7 (3.0)

Chlas/TOC

(µmol g-1)

21.0

8.7

18.0

16.2 (28.3)

Chlbs

(µmol m-2 yr-1)

7.2

1.4

2.0

0.77 (1.0)

Chlc

(µmol m-2 yr-1)

31.5

2.1

1.9

6.1 (0.03)

Fucoxanthin

(µmol m-2 yr-1)

139.2

8.7

12.9

22.3 (0.28)

B) bottom traps

at depth

1.1 - 1.4 km

1.1 - 1.4 km

0.68 - 0.9 km

0.68 - 0.9 km

DM

(g m-2 yr-1)

117.8 N2

141.0N1, N2

109.3S2, N2

18.7 S1, S2, N1

TOC

(g m-2 yr-1)

9.2N1, N2

12.1 N1, N2

5.3 S1, S2, N2

1.7 S1, S2, N1

C/N

atomic ratio

11.2

13.0

11.0

11.2

Chlas

(µmol m-2 yr-1)

203.9 S2, N1, N2

81.2 S1, N1, N2

30.2 S1, S2, N2

6.3 S1, S2, N1

Chlas/DM

(µmol g-1)

1.55 S2, N1, N2

0.64S1, N1, N2

0.26 S1, S2

0.32 S1, S2

Chlas/TOC

(µmol g-1)

20.4S2, N1, N2

6.5S1, N2

5.5 S1

3.9 S1, S2

Chlbs

(µmol m-2 yr-1)

2.6N1, N2

2.5N1, N2

0.82S1, S2, N2

0.40 S1, S2, N1

Chlc

(µmol m-2 yr-1)

5.8 N1, N2

0.93N1, N2

0.26 S1, S2, N2

0.03 S1, S2, N1

Fucoxanthin

(µmol m-2 yr-1)

22.3 N1, N2

5.6N1, N2

0.37 S1, S2, N2

0.17 S1, S2, N1

C) core tops

section

0-1 cm

0-1 cm

time span

yr

~7*

~7**

DM

(g m-2 yr-1)

89*

90**

TOC

(g m-2 yr-1)

3.4

3.5

C/N

atomic ratio

8.5

9.1

Chlas

(µmol m-2 yr-1)

9.9

2.3

Chlas/DM

(µmol g-1)

0.11

0.03

Chlas/TOC

(µmol g-1)

2.92

0.66

Chlbs

(µmol m-2 yr-1)

0.54

0.21

Chlc

(µmol m-2 yr-1)

0.17

0.06

Fucoxanthin

(µmol m-2 yr-1)

1.14

-

* Müller et al.( 2005); ** Mackay et al. (1998), core ´baik29´Note: In the South during both deployments the top trap was exposed at 40 m. In the North the top trap was exposed at 300 m water depth in 2001-2002 and at 50 m in 2002-2003. Therefore, in the North (second deployment) the fluxes and ratios at 50 m were given and those found in 300 m were added in parenthesis. As the fluxes and ratios in the deepest traps varied only little, the lowest four traps were combined as "bottom traps" and median was calculated. They were deployed between 1100 and 1400 m in the South and between 650 and 900 m in the North. Within the bottom traps superscript S1, S2, N1, and N2 indicate significant differences (at 90 %, Mann-Whitney-U-test) to South 2001-2002 (S1), South 2002-2003 (S2), North 2001-2002 (N1), and North 2002-2003 (N2).

Regression models: The dry matter and Chlas fluxes through the water column decreased with the water depth in the South first deployment and North second deployment, while the flux increased in the South second deployment (cf. Tab. 9A,B). However, the flux of original Chla and other labile pigments as well as TOC and generally the ratios of organic compounds (e.g. TOC and Chlas) vs. dry matter, significantly decreased with depth at both sites and deployment periods (Fig. 36, Appendix C - Tab. 5). These results indicated that although unusual high settling material was trapped in the bottom traps of the South basin in 2002-2003, the degradation of the organic compounds was strong in the water column during that period as well.

Fig. 36. Chla fluxes as as well as TOC/DM and Chlas/DM ratios for both sites and both investigated deployment periods. The respective regression equations and their coefficient of determination (r²) are reported in Appendix C - Tab. 5. The water depth at the mooring site in the North and South basins were c. 900 m and 1400 m, respectively; the upper traps of the North mooring 2001-2002 were lost due to technical disturbances.

In the South, the Chla/TOC, Chla degradation products/TOC and Chlas/TOC ratios did not decrease significantly with the water depth and the pheophytin a/TOC ratio even showed significant formation with depth (Fig. 37A, Appendix C - Tab. 6). In the North, in contrast, the Chla/TOC and pheopigments/TOC ratios decreased linearly with depth; only the pheophytin a/TOC ratio did not vary significantly (Fig. 37B, Appendix C - Tab. 6). These differences of the transfer through the water column caused that the decrease of Chlas/TOC ratios were significantly stronger in the bottom traps of the North than of the South (Tab. 9B).

The Chlb or Chlbs/TOC ratios did not show significant decreases with water depth either in the South or in the North, while the Chlc/TOC ratio significantly decreased at both sites (Fig. 37A,B, Appendix C - Tab. 6). Chlc degradation products were not detected. Most carotenoids/TOC ratios showed exponential decreases with water depth in the South, while in the North significant regressions were calculated only for fucoxanthin and diatoxanthin (Fig. 37A,B, Appendix C - Tab. 6). The pigment/TOC ratios decay with water depth was therefore pigment- and site-specific.

Fig. 37. Pigment/TOC ratios vs. depth through the water column of the South (A) and North (B) basins (see Fig.1 for locations). Curve calculations were based on mean values of the two deployment periods (2001-2002 and 2002-2003). The regression functions are given in Appendix C - Tab. 6. Mean values over the 15 (South) or 9 (North) traps were plotted for pigment/TOC ratios, where no significant decay or formation was found.

Comparison between core tops and bottom traps: The trap material in almost all traps was thought to be anoxic because of the dark grey colour, dead Gammarus, the smell of hydrogen sulphide (H2S) and a lack of oxidised ferric iron (Fe(III)), whereas the topmost centimetre of the surface sediment was oxic (Müller et al. 2005). High degradation could therefore be expected. In both South and North the Chlas/DM and Chlas/TOC ratios diminished strongly in the core tops compared to the bottom traps (Tab. 9B,C). Thereby the Chlas/DM ratios diminished stronger (about 90 %) than the Chlas/TOC ratios (70-80 %). Furthermore, whereas the Chla/DM ratio decreased similarly at both mooring sites, the Chlas/TOC ratio decreased much stronger in the North (~90 %) than in the South (70-80 %; Tab. 9B,C). However, the residence time of the settled material within the upper centimetre was similar at both sites (Tab. 9C). Regional variations in the pigment preservation within these core tops were therefore likely.

Tab. 10. Dry matter (DM) and TOC fluxes, C/N ratios, and Chlas fluxes and ratios in the core top (0-1 cm) of nine short cores. Medians from triplicate samples were calculated. The Chla concentrations in the respective regions in summer and the sedimentation rates are given to describe the sites. Abbreviations: B Bay - Barguzin Bay, S Delta - Selenga Delta. See Fig. 38 for locations, numbering refers to sites from South to North within a region.

Regional differences of DM, TOC and pigments at the sediment surface: The top centimetres of seven additional short cores taken across the lake, one in the South, three in the North, two in the Selenga Delta and one in the Barguzin Bay were therefore analysed. The sedimentation rate was highest in the Selenga Delta; however, because the C/N ratio was not higher in the Selenga Delta (Tab. 10) most of the settling organic material likely results from autochthonous production rather than from allochthonous material transported by the river. The Chlas flux into the surface sediment was highest in the eastern South, much lower in southern Selenga Delta and in the central South, and lowest in the North. The Chlas/DM and Chlas/TOC ratios in the surface sediments varied in the order South, Selenga Delta > Barguzin Bay > North (Tab. 10).

Undegraded Chla contributed less than 6 % to the total Chlas in the North and Barguzin Bay and less than 13 % in the South; it was best preserved (19 %) in the western Selenga Delta (Fig. 38A). That intact Chla was found in the surface sediment marked the importance of the sedimentation of living or at least moribund cells. The share of pheophytin was much greater in the North than in the South and Selenga Delta. Lutein and canthaxanthin were the only pigments detected in all North cores, while in the South and at both river inflow sites fucoxanthin and diadinoxanthin were the dominant carotenoids (Fig. 38B). Highest pigment diversity was found in the southern Selenga Delta (Fig. 38B).

Fig. 38. Composition of (A) Chlas and (B) carotenoids within the uppermost centimetre of the surface sediment gathered from nine coring sites across Lake Baikal in July 2001 and July 2002. Stars mark the coring sites of the associated composition diagrams.

Degradation within the oxidised surface layers: Oxidised layers could be well discerned in the short cores due to sharp changes between the upper orange-brownish layer and the greyish layer below (reduced layer; Fig. 39). Strong degradations of the organic compounds occurred within the upper oxidised layers, which spanned 4 to 15 cm. The thinnest oxidised layer was found in the Barguzin Bay and thickest in the cores from the North.

Degradation for the surface sediments is shown exemplarily for the sites below the moorings and for southern Selenga Delta (Fig. 40, Appendix C - Tab. 7). The TOC/DM ratio, the Chla/TOC ratio, pheopigments/TOC ratio and Chlas/TOC decreased in all three cores (Fig. 40) with the sediment depth. The degradation of TOC (when referred to dry matter) was linear in all cores (Fig. 40, Tab. 11). Thereby, higher decay slopes were found in the core from North and lowest decay slopes were found in the Selenga Delta and in the central South (Tab. 11). The decay of the Chlas/TOC ratios was exponential in all short cores (Tab. 11). Hence, the decay with depth was constant and proportional to the ratio left. Again higher decay slopes were found in the North and lowest decay slopes were found in the central South and in the Selenga Delta (Tab. 11) indicating stronger degradation in the North than in the central South and Selenga Delta.

Chla/TOC, pheophorbide a/TOC and Chlas/TOC decreased exponentially in the South and Selenga Delta, while pheophytin a/TOC and pyropheophytin a/TOC decreased linearly (Fig. 40, Appendix C - Tab. 7). In the North, in contrast, all ratios of chlorophylls and its degradation products to TOC decreased exponentially with sediment depth (Fig. 40, Appendix C - Tab. 7, Tab. 11). Chlbs/TOC and Chlc/TOC also decreased exponentially (except Chlc in the southern Selenga Delta, which was not detected at the bottom of the core; Fig. 40, Appendix C - Tab. 7, Tab. 11).

Fig. 40. TOC/DM ratio and pigment/TOC ratios vs. depth in the surface sediments of the (A) South basin, (B) North basin and (C) Selenga Delta. The regression functions are given in Appendix C - Tab. 7. Mean values over the 15 (South, Selenga Delta) or 12 (North) surface sediment slices were plotted for pigment/TOC ratios, where no significant decay or formation was found.

Tab. 11. Regression models for the decay of TOC/DM and pigments/TOC ratios within the upper, oxidised part of short cores.

The composition of total carotenoids did also change with the core depth (Fig. 40, Appendix C - Tab. 7). In the South and Selenga Delta, the dominant carotenoids in the core top were fucoxanthin (77 %) and diadinoxanthin. Other carotenoids contributed less than 2 % to the total carotenoids. However, at 10 cm core depth, fucoxanthin could no more be identified and most prominent carotenoids were diadinoxanthin and lutein. However, in the North, lutein and canthaxanthin were the only carotenoids identified already in the core top and only canthaxanthin was identified in the sediment below. These findings strengthened the assumption of a much stronger degradation in the North compared to the South before final deposition below the redox layer. Thus, as has been shown for the water column processes, the degradation in the surface sediment was also pigment- and site-specific.

In all three investigated short cores (´Vidrino´, ´Posolski´, and ´Contintent Ridge´, located in the South basin, Selenga Delta, and North basin, respectively) a strong decrease of pigment and TOC contents was found within the upper 10 cm (Fig. 41, Fig. 42, and Fig. 43). Those decreases cannot be related only to an increase in the phytoplankton standing crop from former to present time, but with high certainty to strong degradation within the oxic or oxidised layers (cf. chapter 3.2.3). The upper 10 cm will, therefore, not be discussed further in this Holocene study. In all three Holocene cores, original, undegraded Chla that indicated the presence of living algae occurred within the uppermost centimetres, but was not detected below.

Vidrino (South basin): The average atomic C/N ratio was 10.5 and varied little along the core (8.9 % variation from 9 to 12; Tab. 12 and Fig. 41). The average TOC content over the core was 1.5 % and the average Chlas/TOC ratio over the core was 0.3 µmol g-1 (Tab. 12). Both the TOC content and the Chlas/TOC ratio increased from 40 to 60 cm sediment depth (Fig. 41).

The proportion of the distinct Chla degradation products varied only slightly with depth and the contribution of pheophorbide a was low throughout the core (Fig. 41). The Chlbs/Chlas ratio varied without any depth trend (Fig. 41). The mean Chlbs/TOC ratio was 0.03 µmol g-1 with a higher coefficient of variation than that of Chlas/TOC (62 %; Tab. 12). Chlc was not detected in all core depths. Several Chlc/Chlas ratio peaks were found within the upper 15 cm as well as below 40 cm, only one peak was found in between (Fig. 41). The carotenoid/Chlas ratios showed a similar trend to the Chlas/TOC ratio with lowest occurrence between 10 to 30 cm, but towards the bottom of the core greatly increased carotenoid/Chlas ratios were found.

Posolski (Selenga Delta): The average atomic C/N ratio was 9.8 and varied little along the core (6.5 % of variation from 9 to 11; Tab. 12, Fig. 42). The average TOC content and the Chlas/TOC ratio over the core were 1.87 % and 0.4 µmol g-1, respectively, with a coefficient of variation of 1.3 % for TOC and 41.2 % for the Chlas/TOC ratio (Tab. 12). Both the TOC content and the Chlas/TOC ratio showed a long lasting maximum from 35 to 65 cm and were lowest at the bottom of the core (65-70 cm; Fig. 42).

The proportion of the distinct Chla degradation products varied along the core (Fig. 42). The proportion of pyropheophytin a, for example, was higher than that of pheophytin a in the upper 15 cm, but similar on average between 20 and 40 cm and increased again at the bottom of the core (Fig. 42).

The Chlbs/Chlas varied along the core without any depth trend (Fig. 42). However, the Chlbs/TOC ratio was significantly higher at Posolski than at Vidrino. The Chlc/Chlas ratio showed several maxima within the upper 10 cm and another at 47 cm similar to the Chlbs/Chlas-peaks (Fig. 42). The number of detected carotenoids was highest at Posolski compared to the other two sites. The carotenoids/Chlas ratios showed were lowest between 15 and 40 cm and showed then a strong increase from 40 cm towards the bottom of the core (Fig. 42). They did not show similar trends to Chlas/TOC, Chlbs/Chlas, or Chlc/Chlas ratios (Fig. 42).

Continent Ridge (North basin): The average atomic C/N ratio was 7.7 and was significantly lower than that at Vidrino and Posolski (Tab. 12). Two pervasive C/N maxima were found between 20 and 42 cm, and between 46 and 65 cm (Fig. 43). The average TOC content was 1.26 % and was like the average Chlas/TOC ratio (0.1 µmol g-1) significantly lower at Continent Ridge than at Vidrino or Posolski (Tab. 12). The variability, in contrast, was highest for both TOC content (5.7 %) and Chlas/TOC ratio (71.3 %) at Continent Ridge (Tab. 12). The TOC content showed similar trends to the C/N ratio (Fig. 43). Strong maxima of the Chlas/TOC ratio were found at around 50 and 60 cm (Fig. 43).

The distinct Chlas degradation products showed distinct trends with the core depth (Fig. 43). In the 5-20 cm section (during TOC and Chlas/TOC minimum), the share of pheophorbide a was low, but the share of pyropheophytin was high, although not constantly. Below 20 cm, pheophytin was the dominant degradation product, and only at the bottom of the core, did the proportion of pyropheophytin increase.

The Chlbs/TOC ratio was lower at Continent Ridge than at Vidrino and Posolski, like the Chlas/TOC ratio, but the variability was much higher (Tab. 12). The Chlbs/Chlas and the Chlc/Chlas ratios showed several maxima along the core, which did not correspond to the Chlas/TOC maxima (Fig. 43). Canthaxanthin was detected within the uppermost centimetres, at 14 cm depth and below 20 cm, but lutein was detected only in a few samples between 56 and 62 cm (Fig. 43). The carotenoid/Chlas peaks did not correspond to C/N, TOC, Chlas, Chlbs or Chlc versus TOC peaks.

Fig. 43. Holocene high resolution analysis of photosynthetic pigments and organic carbon at ´Continent Ridge´ coring site (North basin). See Fig. 41 for dating and abbreviations.

Comparison of temporal changes at the three coring sites: At Continent Ridge, TOC and Chlas/TOC increased strongly from c. 11 to 9 kyr BP (Fig. 44). The Chlas/TOC ratio then decreased strongly from 9 to 8 kyr BP, was then low up to c. 6.5 kyr BP, and had a clear peak at c. 6 kyr BP. Shortly after 6 kyr BP, TOC and Chlas/TOC ratio decreased towards c. 4 kyr BP. For TOC this minimum lasted up to c. 2 kyr BP. Meanwhile the Chlas/TOC ratio showed a peak at c. 3.5 kyr BP. At Vidrino the TOC and Chlas/TOC ratio decreased from c. 4.5 kyr BP on and this trend lasted for c. 3 kyr. At Posolski the TOC and Chlas/TOC ratio decreased from 3 kyr BP on and this trend lasted for c. 1.5 kyr. All cores showed highest TOC contents and Chlas/TOC ratios from c. 1.5 kyr to present, which are however within the oxidised layer characterised by strong degradation processes (cf. chapter 3.2.3).

At Continent Ridge the Chlbs/Chlas ratio and the Chlc/Chlas ratio varied only slightly during the early Holocene (c. 11 to 8 kyr BP; Fig. 44). From c. 8 to 7 kyr BP the Chlbs/Chlas ratio showed a peak, while the Chlc/Chlas ratio was low. Then at c. 6.5 kyr BP the Chlbs/Chlas was at a minimum and the Chlc/Chlas ratio exhibited a strong peak. Thereafter, the Chlbs/Chlas ratio increased strongly, while the Chlc/Chlas ratio tended again towards zero. Both the Chlbs/Chlas and Chlc/Chlas ratios increased at Continent Ridge sharply again between c. 3 and 2 kyr BP, but both peaks were not detected at Vidrino and at Posolski. At Vidrino a considerable increase of the Chlc/Chlas ratio was noted from 5 to 4 kyr BP, which was, however, not found at Continent Ridge and at Posolski.

Fig. 44. Comparison of temporal changes at three coring sites ´Vidrino´, ´Posolski´, and ´Continent Ridge´ for TOC content, Chlas/TOC ratio, Chlbs/Chlas ratio, and Chlc/Chlas ratio. See Fig. 41, Fig. 42, and Fig. 43 for dated data points and abbreviations. The periods between the dated depths were linearly interpolated. For TOC content, Chlas/TOC ratio, and Chlbs/Chlas ratio averages of two contiguous data points were plotted in order to minimise a possible analytical error; each peak was then supported by several data points. Single peaks for high-resolution contexts can be found in Fig. 41, Fig. 42, and Fig. 43. For Chlc/Chlas ratio data were not averaged because most peaks were detached (cf. Fig. 41-Fig. 43).

Pigments detected in deep sediments:A typical fluorescence chromatogram of the Kazantsevo optimum is shown in Fig. 45 (refer to Fig. 3 for degradation pathway). The terms Chlas or Chlbs mean the sum of all respective chlorophylls, epi- and allomers and degradation products. Steryl chlorin esters (SCE) were also analysed and added to the Chlas or Chlbs.

Fig. 45. Typical HPLC-chromatogram of the Kazantsevo Interglacial showing the retention times of the main chlorophyll degradation products pheophorbide, pheophytin, pyropheophytin and steryl chlorin ester (SCE; refer to Fig. 3 for degradation pathway).

High resolution analysis: The TOC content and the Chlas/TOC ratios was much higher between 560 and 480 cm of this core than in the section below (580 – 560 cm) and the section above (480 – 420 cm), clearly indicating a warm phase during that period (Fig. 46). Assuming a sedimentation rate of 6.4 cm kyr-1 (Demory et al. 2005) this Kazantsevo Interglacial lasted for c. 12.5 kyr (this time is approximate, as the dating was performed on a parallel core).

The C/N ratio was only slightly higher during the Interglacial than in the glacial periods (Fig. 46), and was significantly lower than during the Holocene (Tab. 13). The TOC content increased from the Tazovsky Glaciation to the Kazantsevo Interglacial from 0.3 % to 1.5 %, but was also significantly lower than that preserved during the Holocene (Tab. 13).

The average (centennial) Chlas accumulation rate during the Kazantsevo Interglacial was 2.3 nmol cm-2 100 yr-1. The Chlas accumulation rates showed similar trends to the Chlas/TOC ratios and the TOC content, but the difference between the glacial and interglacial phases was even more pronounced for the Chlas accumulation rates and Chlas/TOC ratios than for the TOC content.

During the preceding (Tazovsky) glacial period negligible amounts of Chlas and Chlbs were detected (<0.1 µmol g-1 TOC), however, during the Kazantsevo Interglacial a strong increase occurred. The Chlas accumulation rates and the Chlas/TOC ratios showed three maxima, one at 535 cm (c. 125 kyr BP), one at 515 cm (c. 122 kyr BP) and one at 505 cm (c. 120.5 kyr BP) (Fig. 46). Even during the ensuing Early Zirianski Glaciation, the Chlas accumulation rates and the Chlas/TOC ratios were not homogenous; both first sharply decreased, but then increased again showing two small peaks (cut-out, Fig. 46). These events clearly indicate that during the glacial phases short and slight warmings occurred.

The Chlbs/TOC ratios were significantly lower during the Kazantsevo interglacial than during the Holocene (Tab. 13). However, the Chlbs/Chlas ratios increased strongly from the Kazantsevo Interglacial towards the Early Zirianski glacial period, indicating increasing importance of Chlorophyta (Fig. 46).

Fig. 46. High resolution analysis of lipophilic photosynthetic pigments and organic carbon during the Kazantsevo Interglacial at ´Continent Ridge´ (North basin). See Method section for dating, and Fig. 41 for abbreviations. SCEs (steryl chlorin esters) were included within the term ´Chlas´ and ´Chlbs´.

During the late Tazovsky Glaciation pheophorbide a contributed only 20 - 40 % to the total Chlas, while SCEa contributed up to 60 %. During the early Interglacial, pheophorbide a and SCEa showed similar contributions, but during the late Interglacial the pheophorbide a contributed more than 50 % to the total Chlas. During the Early Zirianski Glaciation the SCEa clearly dominated again.

Several forms of pheophorbides, pheophytins, pyropheophytins and SCEs were found whereby some occurred also during the Holocene (Fig. 47). Depth trends were found during the Kazantsevo and clear distinctions between the Kazantsevo and the surrounding cold phases. Similar to the Holocene (Continent Ridge core), the dominant pheophorbide a was pheophorbide a8+9 during the Kazantsevo, but pheophorbide a11-12 and a13-14 were also found in considerable amounts. They were scarce, in contrast, during the cold periods. Pheophytin a20-21 occurred at the top of the Holocene core as well as during the warm Kazantsevo but also during the Chlas maximum of the Early Zirianski Glaciation. In contrast, pheophytin a21.1-22.1 was found during the Kazantsevo optimum, but only in low amounts during the glacial periods. The distinct pyropheophytin a forms did not show depth trends during the Holocene, but during the complete Kazantsevo and its transitions pyropheophytin a24.5-25.2 dominated with more than 95 %. The SCEa forms identified in the Kazantsevo segment, showed periodicities, which were, however, not related to Chlas maxima or to warm and cold periods.

Perylene: Perylene was detected in the three Holocene cores and also in the Kazantsevo segment (Fig. 48). At Vidrino and at Posolski perylene peaked at around 0.5 kyr BP. In the North (Continent Ridge), in contrast, perylene was not detected or only in traces since 4 kyr BP. Major peaks in the North were found before 7 kyr BP. No significant correlation of perylene with any of the photosynthetic pigments or with TOC was found. Furthermore, perylene was not detected during the Tazovsky Glaciation, peaked during the early Kazantsevo Interglacial, was again not detected during the late Kazantsevo Interglacial, but peaked again during the Early Zirianski Glaciation. Therefore, the occurrence of perylene in Lake Baikal could not be attributed to any region or to climatic changes.

Fig. 48. Temporal changes in perylene concentrations in the (A) Holocene (three sites) and (B) in the Kazantsevo Interglacial (one site). Perylene (in relative units) was normalised to one, whereby one is the highest content of perylene found within the respective core. The highest content of perylene in the Vidrino core was 25 fold higher than that of the Continent Ridge core and three fold higher than that of the Posolski core.